It is known to protect the measuring arm of a coordinate measuring apparatus against collision with the workpiece or with operating personnel. Protective arrangements of this kind are disclosed, for example, in U.S. Pat. Nos. 5,038,488 and 5,269,068. In these known collision protective arrangements, a bellows completely covering the measuring arm is utilized to activate the protective arrangement. The bellows includes special cutouts in its struts through which light beams or electrically-conductive wires are guided. The light beams are interrupted by the bellows or the wires come into contact therewith when the bellows is displaced by an obstacle. This interruption or contact generates a switching signal which stops the movement of the machine.
These known solutions are relatively complex and require a bellows surrounding the measuring arm, but this bellows does not necessarily have to be provided in each case. Coordinate measuring apparatus are available in a great many different configurations and types and the measuring arm carrying the probe is not necessarily movable in a purely linear direction. For example, U.S. Pat. No. 5,396,712 discloses a coordinate measuring apparatus of the so-called pivot-arm type wherein the measuring arm, which carries the probe, is rotatably journalled about two axes. The extended length of the measuring arm of this type of apparatus does not change and therefore there are also no guides which are exposed and would have to be protected against contamination. For this reason, the apparatus has no bellows. On the other hand, the pivot arm of the apparatus is linearly movable in the direction of the pivot axes and is caused to motorically track the deflections of the probe in this direction This then presents the problem that the pivot arm can run up against the workpiece and become damaged in the course of this tracking movement.